The present invention relates to an apparatus for the manufacture of corrugated paperboard and, more particularly, to a heat transfer control system for a double backer in which the corrugated web is more effectively and efficiently dried and the adhesive uniformly cured.
In a typical prior art double backer (also commonly called a double facer), a single web is brought into contact with the glued flute tips of a single face corrugated web and the freshly glued double face web is passed over the surfaces of a number of serially arranged heating units, usually steam chests, to cause the starch-based glue to set. Double face web travel over the flat heated surfaces of the steam chests is provided by a wide driven holddown belt in direct contact with the upper face of the corrugated web and the top face of the belt, in turn, is held in contact with the traveling web by a series of ballast rollers or the like, all in a well known manner.
Prior art steam chests, one example of which is shown in U.S. Pat. No. 3,175,300, are typically made of heavy cast iron or steel in the manner of a pressure vessel in order to contain the high pressure steam which is supplied to heat the steam chest. For example, the walls of a cast iron steam chest are typically 1 inch (2.5 cm) or more thick to safely contain superheated steam supplied, for example, at 350.degree. F. (177.degree. C.) and 160 psi (1103 kPa). A steam chest heating unit typically has a flat upper web-supporting surface having a length in a transverse cross machine direction sufficient to support the full width of the traveling web. The width of the steam chest in the direction of web movement is typically about 18-24 inches (47-61 cm). Eighteen identical steam chests may typically be serially arranged in closely spaced relation in a double backer.
The heavy metal construction of steam chests results in a number of well known operational problems. The heat transfer rate from the heated surface of the steam chest to the paperboard web is maximized when intimate contact between the two is maintained. Conversely, the heat transfer rate diminishes dramatically if even air gaps as small as 0.001 inch (0.025 mm) exist. The moving corrugated web carries on its surface a laminar boundary layer of air which insulates the web from the heated surfaces of the steam chests over which the web travels through the double backer. In order to raise the rate of energy transfer to the corrugated web, various systems are known in the prior art to provide a ballast load to press the belt and corrugated web against the heated surfaces of the steam chests. These ballast systems include relatively simple roller arrangements, some of which include means for applying a variable downward force, and relatively more complex compliant weighting systems, such as inflatable air membranes. Examples of such systems are shown respectively in U.S. Pat. Nos. 5,183,525 and 5,256,240. In a typical double backer, however, the spacing between the steam chests creates a gap ranging from about 1/2 inch to 1 inch (about 1.25-2.5 cm) between adjacent units. This gap allows the insulating laminar boundary layer of air to effectively re-establish itself between each steam chest.
It is known in the art to place air nozzles and steam nozzles in the gaps between steam chests, as shown for example in U.S. Pat. Nos. 2,987,105 and 3,226,840. In the former patent, the air nozzles assist in cooling and lifting the corrugated web from the heated surfaces. In the latter patent, steam is applied to assist in drying the web and curing the adhesive. U.S. Pat. No. 995,084 discloses a double backer in which the corrugated web is captured between two traveling belts each made of an open-mesh wire cloth. Vacuum chambers are spaced along the surface of the hot plate system and vacuum is drawn through a foraminous plate which closes the top of each chamber to draw off air and moisture. However, the corrugated web remains spaced from the heated surfaces of the steam chests by the lower wire cloth belt.
Another problem directly affecting the efficiency of heat transfer to the web from the steam chest is the transverse bowing of the upper heating surface during operation. The temperature of the flat upper surface of the steam chest is reduced substantially relative to the bottom wall of the steam chest as a result of the cooling of the top surface from contact with the corrugated web traveling thereover. The difference in thermal expansion between the top and bottom surfaces of the steam chests produces a warp or concave bowing of the upper surface lengthwise of the steam chest (transversely across the web). Ballast systems, particularly the compliant loading systems, are intended to remove the air gap by forcing the holddown belt and paperboard web to follow the bowed contour, but in operation provide a number of disadvantages. First, drag on the holddown belt is significantly increased, which in turn causes accelerated wear of the belt. Also, the drag reduces the life of the drive system. In addition, excessive normal force on the holddown belt can result in crushing of the corrugated web and a resultant adverse effect on corrugated board quality and performance. Accelerated holddown belt wear is more pronounced on the outer edges of the belt where the belt contacts the metal heating surfaces when less than full width web is being processed.